I am curious about the Daedric Princes and being able to exist within Nirn. Throughout Skyrim and the quests I have done for the Princes I have noticed that only Sanguine interacts with you in a physical form, all other princes either contact you telepathically or from the use of an avatar.

Anyway my question is, what are the requirements for a Daedric Prince to enter Nirn? For example could it be worship, sphere, power level or summonings?

It's mostly a "we really can't do anything harmful, just dike around in avatars." Something akin to the gate in the Shivering Ilse. If you noticed, they really only egg you on or tell you to do something.

Good question, but we only saw Sanguine's avatar. Daedric princes are actually the size of planets. Or, infinite kinda.

Requirements ? See the plot of oblivion to give you a general idea. Most princes wouldn't be interested in physically entering Nirn.


This.

Indeed, what we think of as the princes are really just meat-puppets they drag out for the benefit of mortals.

Just imagine if Mars, the planet, whispered in people's ears, telling them do all kind of heinous mars-y stuff.

Mars is the planet of war.

Nocturnal's appearance wasn't a ghost aspect like most other Daedric Princes, but that is easy to explain since she rises out of the portal to the Evergloam.

Also, Azura appears in person in Morrowind.

Azura has been smoten unto as any void-aperture can be smote at http://www.imperial-library.info/content/trial-vivec.
That opened my eyes, at least.
None of the aspects can be seen as evil, nor can they be seen as good. They are not whole, they are fragments of what can be aspired to.
Azura depicts herself as fate, though it is not her realm she presumes preknowledge over, she is the desire to see Dawn unto Dusk, she is the want for safety, predictability, man's desire to see a cave with no bears in.

You and Mars dude.

Remind me, which wars has Mars participated in again?

Mars is one of several combatants in the gravitational disputes with Jupiter over the Asteroid belt.

I'm not sure this is still as true as it was, and/or it is becoming less true. The towers are falling, after all.

While the daedra in Skyrim still manipulate mortals into doing most of their dirty work for them, they do seem to be manifesting in more tangible ways on Nirn. In Oblivion, they could speak to you via their shrines and gift you their artifacts, but that was it. No mortal forms walking around, not even spectral avatars. In Skyrim, however, Sanguine has a meat puppet, Barbas is flesh, Dagon has dremora guarding his shrine, Nocturnal sends some sort of Avatar through her portal, Boethiah takes possession of a corpse, and even Peryite shows up as a ghost-rat. Many of the Princes seem more capable of sticking their fingers through the fabric of Mundus than they had been before.

Also, Molag Bal flings around house furniture and locks doors.

Barbas was flesh in Morrowind, too (the Creeper). Dremora also guarded various shrines, and would appear if you tried to steal certain things. Azura showed up as a ghost. Hircine was also able to show up physically (as he had done every thousand years or so prior).

In Oblivion, Molag Bal could resurrect you, and Azura could unlock doors.